I have had an echeque from a buyer not clear in the past. It appeared as cancelled. When I questioned Paypal as to whether the buyer had cancelled it, they advised me that buyers cannot cancel an echeque. They suggested that the reason it was cancelled was that there were insufficient funds in the buyer's bank account.

You should contact the seller, and if they have a change of mind refund policy, request a refund once they receive your payment and a mutual cancellation. It is inappropriate not to bother to advise the seller that you no longer want the item.